For the vehicle to operate autonomously, a gear changing system needs to be included on the vehicle. If the vehicle has an automatic gearbox then this is not necessary, assuming it has a method of setting drive, reverse, neutral, and park. Otherwise, if the vehicle has a manual gearbox then a gear changing mechanism needs to be implemented to allow the system to perform the necessary gear changes, including the clutch control to engage the motor and gearbox.
The TRX-300 quad bike has a manual gearbox, with a gear changing sequence in the following order; reverse, neutral, super low, then first through to the fourth and final gear as shown in Figure 5-32 below.
The TRX-300 quad bike features a centripetal clutch, which when the engine reaches a certain RPM, the centripetal force acting on the clutch forces the clutch shoes into the clutch housing engaging the engine drive shaft with the transmission shaft. Because of this, the clutch will automatically engage with the gearbox once the engine has reached a speed that is capable of driving the gearbox without stalling. If the engine RPM decreases below the point at which it engages with the gearbox, the clutch will disengage and thus will prevent the engine from stalling if the engine load is too high to be sustained by the throttle input.
To provide the necessary functionality to operate the gear system, the gearbox mechanism needs to be capable of the functions: changing up and down gears, detecting/recording which gear the gearbox is currently set to and providing a fast change time that is as fast as the change time for a human operator. Finally, to maintain the manual control capably of the system, the user should be able to manually changing gears using a method that is similar to the existing gear changing foot pedal. This is particularly necessary in the event of system brake down to prevent the quad bike from becoming unusable.
To prevent accidental gear change from neutral to the reverse gear a reverse activation mechanism needs to be actuated. This is traditionally accomplished using the rear- braking handle on the left hand side of the handlebars. To do this a secondary pushpin must be pressed to couple the reverse cable to the braking leaver; this ensures the quad bike is stationary when the reverse gear is engaged. In order for the control system to be capable of engaging the reverse safety mechanism and in turn engage the reverse gear, a mechanism is required to actuate the reverse cable and is covered in section 5.5.3. Alternatively, the reverse locking mechanism could be modified to ensure that the mechanism is permanently engaged. Due to safety concerns and to prevent possible damage to the gearbox, this modification to the gear system was not undertaken.
The TRX-300 features three-indicator lamps in the centre of the handlebars, as seen in Figure 5-33. These indicator lamps provide a visual method of detecting if the gearbox is in the neutral position or if the reverse gear is engaged. It also provides a warning light for the temperature of the engine oil.
Through examination of the wiring diagram contained with the manual for the TRX- 300, it was determined that the indicator lights work using a set of switches that when active, complete the lamp circuit by grounding the negative terminals of the lamp causing them to illuminate, as shown in Figure 5-34 Indicator light activation mechanism.
Figure 5-34 Indicator light activation mechanism
5.4.1
Design Considerations
The main objective of this system is to provide a method of changing gears on the quad bike without user intervention, while still allowing the user to change gears manually using their foot, when manual changing is required or preferred. After consulting with several of the engineering technical staff at Massey University, two designs that were considered; the first was to use a set of two spring return solenoids that when energized would either pull the gear leaver up or down. As long as the solenoids provided the necessary travel distance at the position at which they are coupled to the gear changing leaver, and can apply the required force at this position, then they would be capable of changing the gear over a very short period.
Depending on the size of the return spring used, most solenoids requiring less than a second to move from limit to limit under no load conditions. The solenoids used would require a long travelling stroke, with the gear leaver positioning the solenoids at around the centre of their travel stroke. This would allow the gear leaver to travel in both directions freely, as well as allowing the same freedom of motion if used manually, assuming the solenoids remained unpowered.
The second design featured a leaver arm on the gear pedal, which uses an actuation wheel to manipulate the arm. This design uses a motor to rotate an actuation drum to apply a force to the leaver arm mounted on the gear-changing pedal, this drum features two wheels made out of bearings, shown in Figure 5-35. These pusher wheels reduce the friction between the drum and the leaver arm. During actuation of the leaver arm, the point at which the arm and drum contact one another changes, as the angle on the leaver arm changes. The actuation drum, driven by a motor, requires a positional sensor so that a controller can know the relative position of the wheel to the leaver arm.
Manual gear changing can still be achieved by returning the actuation drum to the neutral position after each gear change, by positioning the pusher wheels on the actuation wheel at a distance that allows the leaver arm to move past the pusher wheels when in a set position. This neutral drum position shown in figures Figure 5-35 and Figure 5-36 shows the movement of the leaver arm (green) past the bearings mounted on the actuation wheel (red).
Figure 5-35 Neutral Drum position 1 clearance
Figure 5-36 Neutral Drum position 2 clearance
Using either method will require a method to detect the gear change, the neutral and reverse gears can be detected using the indicator lights available. The TRX-300 does not feature any inbuilt method for detecting when a gear change has occurred, apart from the fore mentioned indicator lights. The solution for this problem uses limit switches to detect when the gear changing leaver has passed the position at which gear changing occurs, and thus a pulse can be sent to the controller circuit that will indicate a gear change has occurred.
5.4.2
Torque Measurements and Calculations
To measure the torque required for actuation the gear leaver and successfully change gears, a set of weights were used to measure the minimum force at which a gear change occurred. These weights were used instead of the portable scales used in previous experiments, due to the gear change arm requiring negligible force to be applied to manipulate the gear leaver once a gear change has occurred.
A 2kg, 5kg and multiple 50g sets of weights were used to apply the necessary force on the gear leaver. Using these weights, the smallest weight capable of triggering a gear change from super low to neutral was seven kilograms. With the force from the seven-kilogram weight guaranteed to trigger a change. The weights were positioned at a point that was 185mm from the pivot point, resulting in a torque of 12.7 Nm applied to the gear leaver.
5.21
Due to the difficultly to measure a down change in gears, where the gear leaver must be moved up, it has been assumed that the force-required move the leaver up is similar to force required to change the gearbox to a higher gear. Thus, the minimum torque required to change gear in the gearbox was been calculated as 12.7 Nm.
To determine the torque required to operate the pusher wheel, the position at which gear change is achieved must be determined. The vertical displacement from the neutral gear leaver position and the point at which it changed was 50mm and 53mm for the up and down gear change respectively. These measurements were taken from at a chosen point with a distance of 176mm from the pivot point. This gives the angle of 16.5 degree for the up when the gear leaver in up, and 17.5 degrees for the down position.
5.22 5.23
Figure 5-37 shows a 3D model built in SolidWorks to test the viability of the proposed system using an image of the space behind the gear leaver. This image was scaling to allow for a 1:1 3D model of the physical mechanism and position the leaver arm and pusher wheel correctly.
Figure 5-37 SolidWorks model (shown left) to simulate gear mechanism (original image right)
Using this, the motion of the leaver arm and pusher wheel was modelled, with the angles for the up and down gear change modelled to determine the point where the leaver arm and pusher wheel make contact. The centre distance between the wheels on the drum and the drum pivot point is 41mm, and the point at which the leaver and pusher wheel contact
was measured by setting their relative positions. When system changes down a gear, the drum and leaver arm come into contact at a point 66mm from the pivot point of the gear arm, with a 76mm length measured for changing up a gear.
From this model, the point at which the maximum force will need to be applied is where the leaver arm will be at its shortest length; in this case, it will be when the arm is 66mm in length when the system is changing down a gear. From this, a force of 192.4N will be required to act on that point to achieve a torque on the gear leaver equal to 12.7Nm.
5.24
Therefore, from this a torque of 7.9Nm is required from the motor driving the pusher wheel, which has a leaver arm of 41mm.
5.25
Using windscreen motor, the same method as described in section 5.2.1 was used to calculate the torque, with a final calculated stall torque of 13.66Nm. From this, it was concluded that the available motor would be more than sufficient to operate the gear changing mechanism, with a safety factor of at least 1.73. This safety factor is based off the estimated torque required to operate the gear leaver, with the actual minimum torque required to trigger a gear change between 9.26 Nm (5.1kg weight) and 12.70 Nm (7 kg weight). This torque range will compensate for any discrepancy in the positioning of the mechanism, in regards to the leaver arm distances not matching the model.
5.4.3
Integration and Testing
To ensure the system was positioned to match the Solidworks model, a guide plate was fabricated out of 2mm aluminium to ensure the centre points of the motor and gear shaft matched. With the centre points matched, a steel plate was used to mount the aluminium motor mounting plate, which was then wielded into position. A leaver arm fabricated out of 20mm steel plate and wielded to the existing leaver arm. This thickness provides an ample width for the 6mm drum wheels in case the actuator drum is not parallel to the leaver arm.
To test the functionality of the gear changing mechanism, the motor was run in both clockwise and counter clockwise directions to attempt gear change. The results of this test showed that the mechanism was capable of actuating the gear leaver. This confirmed that the earlier assumption in which the force required to actuate the gear leaver up was equal to or less than the force required for a downward actuation. However, this did reveal a problem that over rotation of the pusher wheel past its digital limits will cause the system to physically jam, within the mechanism disassembly required to return it to normal operation. This should not be a problem as long as the control system functions correctly.
Figure 5-38 A hydraulic jack was used to set the gear change point